The invention relates to the field of decontamination of soil and other materials and a method and apparatus for accomplishing the purification of soil and other materials contaminated with organic materials, and in particular to the purification of soil and other materials contaminated with both higher and lower volatility organic materials.
Increasingly, the public has been made aware of some of the less desirable side effects of industrialization in the form of industrial wastes and other environmental pollutants. While neither the magnitude of contamination nor the effects of the pollution are known with any degree of certainty, it is generally accepted that returning the environment to a pristine state is highly desirable.
One area of particular concern is soil decontamination. Through years of thoughtless disposal practices, large areas of soil have become contaminated by a variety of pollutants. Among the most prevalent classes of pollutants are the Volatile Organic Contaminants ("VOC"). VOC's are organic compounds or mixtures of organic compounds which can range from low boiling point, low molecular weight primary alkanes, alcohols, amines, amides, acids, sulfides, etc. through high boiling point, high molecular weight compounds such as dioxins and PCB's.
The removal of VOC's from soil has recently become the subject of a number of United States patents which are directed generally to methods and apparatus for soil decontamination.
For example, Noland, U.S. Pat. No. 4,738,206 relates to an apparatus and method for the low temperature thermal stripping of volatile organic contaminants from soil. The method involves the sealing of contaminated soil within a conveyor system and vapor stripping the contaminants. This is apparently accomplished by heating the contaminated soil, which is retained under a negative pressure, by use of a hollow screw conveyor through which hot oil is passed and sweeping the combustion gases from the oil heater over the soil as it is transported by the conveyor, so that a temperature gradient is formed between the devices inlet and outlet.
However, several problems exist with this technology. First, considering the negative pressure and the relative orientations of the combustion gas inlet and the contaminant-containing gas outlet, it is unlikely that the desirable temperature gradient can be formed throughout the entirety of the sample being treated. Furthermore, the use of hollow screw conveyors is problematic. Hollow screw flights are costly to fabricate. There is always a danger that the hot oil will leak out of the system at any of the complex joints at the ends of the conveyor. This leaking fluid may reduce the efficiency of heat transfer, threaten the mechanical integrity of the device and create a new source of potentially toxic environmental pollution. In addition, because of the thermal properties of available heating fluids, the range of temperatures achievable through the system is dramatically reduced. Furthermore, the pressure drop created by pumping the heat transfer medium through the screw limits the flow required for effective heat transfer to the soil. Accordingly, the applicability of this technology is limited to relatively low boiling point VOC.
Welsh, U.S. Pat. No. 4,702,178 relates to an emergency exhaust system for a hazardous waste incinerator. While the majority of this patent discusses the eventuality of a malfunction in one of a plurality of incineration systems, the patent does disclose the use of a belt-type conveyor such as the type disclosed in U.S. Pat. Nos. 3,648,630 and 4,202,282, for conveying contaminated material under a series of rakes and infrared heating elements, wherein solid waste material is oxidized into gaseous combustion products which are collected through a secondary system.
Belt conveyors, however, are large and often inefficient. These conveyors also tend to suffer from the loss of conveyed material which may fall off of the belt conveyor prior to complete treatment. The collection and clearance of this inadvertently discharged material could require the shutdown of the apparatus with resulting loss in time and efficiency. Further, this type of conveyor system may not provide sufficient agitation of the contaminated material to allow for efficient and effective removal of substantially all of the volatile contaminants contained therein. Finally, conveyor belts may be subject to degradation when constantly exposed to the elevated temperature and contaminated atmosphere within the apparatus, and are subject to frequent breakage. The disposal of these conveyor belts may also present its own unique problem.
DesOrmeaux et al., U.S. Pat. No. 4,606,283 relates to a system for extracting contaminants and hydrocarbons from cuttings waste in oil well drilling. This is accomplished by the use of an auger disposed substantially throughout the length of the chamber which is jacketed by a series of electrical heating rods which are used to volatilize drilling oil contained on the cuttings. However, the use of an electrical heating rod external to a metal jacket which surrounds the oil drilling cuttings and the auger is inefficient and wastes considerable power making its operation costly. Second, the patent relates to a very specific method and apparatus designed wholly for the removal of specific organic materials contained in liquid form, namely, cutting oils. The nature and composition of these oils are known to those of ordinary skill in the art and, therefore, disposal methods and apparatus may be easily tailored to the specific requirements thereof. However, these methods and apparatus may be of little or no use in treating soil containing a broad spectrum of VOC.
Hobbs et al., U.S. Pat. No. 3,648,630 discloses an incinerator for burning sludge from waste-water treating plants in which the sludge is initially pre-treated by being heated in an infrared radiated screw conveyor to vaporize a substantial portion of the moisture in the sludge, thereby breaking down the sludge into particle sizes suitable for incineration. According to the specification, at column 6, lines 56-61, the fundamental purpose of the infrared heating in the screw conveyor is not to remove moisture but rather to fragment the sludge, which is tightly bonded together by the interlacing of hair and other filmous particles. There is no disclosure that an infrared heated screw conveyor can be used to heat soil to remove higher and lower volatility organic contaminants without combining the conveyor with other methods of soil decontamination such as soil incineration.
Other methods and patents of interest include: U.S. Pat. Nos. 4,009,667; 4,140,478; 4,231,304; 4,338,869; 4,667,609; and 4,802,424.